This has been suggested numerous times in the past and denied. It's just not something Paul would like implemented due to various reasons. Personally, I think the Like system could promote bad behavior due to bad posts being "liked" by certain users, which would encourage more and more of those bad posts. Also, not to mention, this isn't a social networking site, and this kind of system wouldn't really benefit a trading forum since it is geared for a social networking scene.
Also have a look at
How To Post A Site Suggestion for future suggestions.
